2025 06 18 WEH Slope Stability Analysis Letter Report

Global Stability Study for Eastern Slope near Building D West Essex – Highlands Development West Orange, New Jersey Langan Project No.: 101049801 14 January 2025 Revised 18 June 2025 Page 3 of 5 During our subsurface investigation, groundwater was observed in only one test pit (TP-1), at a depth of roughly 11 feet below existing grade, which is below the top of decomposed basalt. There was no indication of mottling of soils in the test pits that would suggest a seasonal water table. All test pits, excluding R-1B, excavated during the wet season in April, similarly did not encounter water nor did the soil exhibit mottling. Minimal water seepage was observed on top of the rock surface that was encountered approximately 2 feet below existing surface grade. Geologic Hazard Review and Rock Outcrop Stability The Subject Slope has a few scattered rock outcrops along the top and upper portions of the slope. No adverse / problematic jointing was observed. The potential for rock fall hazard is low and would be expected to ravel directly to the ground beneath the outcrops. A high-angle fault with no history of activity is mapped about 2,000 feet to the east of the site. A copy of the geologic mapping is provided as Figure 2. The seismic risk at the site is considered low based on our review of available New Jersey earthquake history in the area. Nonetheless, we included a seismic case in our slope stability analysis. The state of New Jersey published a Landslide Susceptibility Map for Essex County dated 2001, which uses landslide classes from the HAZUS User Guide (FEMA) Table 9.5. One area, identified as Landside Class B III, appears on the map which closely correlates to areas of outcropping shown in Figure 1. We evaluated global slope stability separately from rock stability, which is governed by rock jointing. Due to the typically favorable joint orientation in the outcrops, the rock is stable. Global Stability of Slope Our analysis included an evaluation of slope stability under both static and seismic (dynamic) design conditions. Building D is far enough from the top of the slope that there is no surcharge load that would impact the slope stability, nor will transient automobile loading on the emergency access road connecting the Howell Drive cul-de-sac to the development impact slope stability. Although no indication of a continuous groundwater table was

transient automobile loading on the emergency access road connecting the Howell Drive cul-de-sac to the development impact slope stability. Although no indication of a continuous groundwater table was observed in the test pits, we performed a sensitivity analysis to determine if changes in groundwater level would impact the stability of the subject slope. Analysis Sections A-A’ and B-B’ Our base case static and seismic analyses used the base groundwater level encountered in our one test pit. The results of the static case indicate the lowest factors of safety exceed 1.5, indicating stable slopes. The potential slip plane surface that resulted in the lowest factor of safety was limited in length to roughly 100 feet and occurred near the middle of the Subject Slope as a shallow surficial slip surface (i.e., within the upper 2 feet of soil). Similarly, the results of the dynamic (seismic) case indicate the lowest factors of safety to be greater than 1.2, indicating stable slopes during a potential earthquake. Factors of safety greater than 3.29 and 2.23 were computed where software-generated potential slip planes extend from the top of the Subject Slope to the downslope neighboring properties for the static and seismic cases,